The present invention is directed to a sensor for indicating orientation of a satellite on which the sensor is mounted, and more particularly to an earth sensor. Sensors of the type in accordance with the present invention are generally mounted to satellites, space probes and the like, for sensing the horizon of the earth, and producing a signal indicating orientation of the satellite with respect to the earth around which the satellite is orbiting. The sensors are provided for attitude control and guidance of satellites. The attitude of a satellite is determined by its position with respect to three axes at right angles to each other. Typically, two of these axes are in a plane at right angles to a projected radius of the earth passing through the satellite and which are defined as pitch and roll, and the third axis coincides with a radius defined as yaw.
Earth sensors of the type generally known to the art are most efficient and economical when operating in the visible light region. However, earth sensors operating only in the visible region cannot easily maintain attitude control of the satellite during the portion of the orbit during which the sun is behind the earth and operation in the visible region is not possible. The use of a sensor operating in the IR region will correct this problem, but result in other disadantages including limited achievable accuracy over a wide altitude range as a result of inherent errors in the use of certain types of simple static IR earth sensors. Accordingly, attitude control of a satellite by either a sensor operating in the visible region or a sensor operating in the infrared region each result in disadvantages over the path of an entire orbit of the satellite.
It is the primary object of the present invention to provide a combined sensor exhibiting the advantages of visible sensing means throughout the entire orbit of the spacecraft, and minimizing errors resulting from the use of infrared sensing means during portions of the orbit when visible sensing means are inoperative.